Field of the Invention
The present invention relates to a forward error correcting code encoding equipment, a forward error correcting code decoding equipment and a transmission apparatus with use thereof, in particular, being suitable for an optical transmission network.
Following the development of technologies in the field of digital signal processing, including such as a LSI and so on, the technologies of encoding and decoding of forward error correcting code are applied to or used widely in various technical fields for the purpose of ensuring high quality of signals. In particular, in an engineering field, a code, such as a so-called systematic code, is mainly used due to transparency of the information thereof, among the block codes having mathematically well-regulated systems therein. In this code, a series of continuous signals are divided into a plurality of constant blocks, and are coded or encoded by each block, wherein it is characterized that only a check bit is added into a vacant area predetermined within the signal, but no operation is made on the original signal information. Of this kind of block codes were previously well-known a Hamming code, a BCH code, and a Reed-Solomon code, etc., and they are actually applied to. Hereinafter, the coding or decoding of the forward error correcting code is called only by coding or decoding.
Then, for a system introducing the forward error correcting code therein, it is established with an assumption of deterioration of signal quality due to mixture of noises in each of processes, including reading and reservation of signals, signal transmission, signal processing, etc. For example, in such a manner that the signal is always coded in the each process, and then it is always treated by decoding process thereafter.
Optical transmission is widely spread in recent years, which enables data transmission with a large volume or capacity thereof. This optical transmission adopts optical fibers, i.e., a transmission path of relatively high quality, as a medium thereof, having a bit error rate equal or less than 10−9, and therefore it is not necessary to presume the adoption of the forward error correcting codes in the system. As a representative one of such the optical transmission can be listed a digital synchronization transmission system, wherein regulations are existing worldwide. This system is widely spread in the networks of truck or main line systems of the world, as SDH (Synchronous Digital Hierarchy) (established 1988) which was determined as the recommendation G.707 and so on by International Telecommunication Union (hereinafter, ITU-T), and as SONET (Synchronous Optical Network) (established 1991) which was determined as the standard T1.105 by American National Standardization Institute (hereinafter, ANSI). Those standards, except for a submarine transmission system wherein signals must be transferred very long distance, are not positive nor aggressive for adoption of the forward error correcting code.
However, following advances on the large capacity with time division multiplex in the digital synchronization transmission method, the signal is narrowed in the bit length-and is easily influenced by an affect due to various dispersions, as the inherent physical characteristics of the optical fiber, and as a result, the signals are deteriorated in quality thereof, as well as the transmission distance is limited therewith. Then, as a means effective to compensate for the deterioration of the signal quality, the forward error correcting code is studied. As an example of the code, in Japanese patent Laying-Open No. Hei 7-280058 (1995) is shown the Reed-Solomon code for a single error correction.
Further, on the other hand, a method of wavelength division multiplex is also spread widely, but since a degree of separation is deteriorated when the lights transmitting in a piece of optical fiber come to close to each other in the distance of wavelength thereof, the transmission distance is restricted thereby when the degree of multiplication is increased. In this instance, also introduction of the forward error correcting codes is an effective method for compensation thereof.
It is not necessarily true that in the optical transmission network, such as nodes. (multiplexing equipment and regenerating equipment) constructing the digital synchronous transmission network, always support the same forward error correcting codes. Namely, all of the nodes are not necessarily supplied by the same producer or manufacturer, and also there can be a node (s) which cannot support the forward error correcting codes even if it(they) is supplied by the same producer or manufacturer, depending upon the difference of time of purchasing thereof.
In this manner, under the situation where there are mixed nodes being different from one another in the transmission network, it cannot be guaranteed that the signals received at any node is necessarily coded, and therefore it is impossible to execute the decoding always at the receiver side. This is because miss corrections are generated in the signals when the signals which are not coded properly are decoded.
Further, in general, by using the forward error correction, since miss corrections come to be large in the number when the error rate of the signals is high, there can be considered a method in which the decoding is performed only when the error rate is relatively low, while it is not performed when the error rate is deteriorated. Alternatively, with a transmission medium of relatively good quality, such as the optical fibers and so on, there also can be considered a method of using thereof, in which the decoding is performed only when the error rate is deteriorated in such degree that it does not cause the miss correction, but the decoding is not performed under other situations.
From the above, the decoding equipment need not only ON of the decoding operation (i.e., decode ON) but also a process of not decoding (i.e., decode OFF), and also there is a necessity that a network management system or an operator must setup ON/OFF of the decoding operation for each node of the network.
Regarding the coding, since it is systematic code, there is given no ill influence even if it executes the coding operation always, irrespective of the opposing node(s).
As an example of supervision of the above-mentioned error rate, in the transmission networks such as the SDH or SONET, the number of error bits is detected for each of multiplex sections and for each of regeneration sections on the basis of BIP (Bit Interleaved Parity) of B1 byte or B2 byte. Therefore it is possible for the network management system or the operator to capture the number of error bits and the bit error rate between the respective nodes.
Next, if the transmission network comes to be complex or is altered in the construction thereof, it is difficult for the network management system which is managing the transmission network, to grasp in which node section the error correction is applicable or not, or to make an instruction. to execute the error correction. This may sometimes cause that non-encoded signals to be erroneously decoded.
Further, in the decoding processes, there is caused delay time corresponding to one coding block or more than that. Here, explanation will be given on generation of the delay time by the decoding processes under the decode ON condition, by referring to FIG. 7 showing a timing chart for an explanation of generation of the delay.
The data inputted into a decoder is outputted therefrom, with a time delay “Tm”, as indicated in FIG. 7. Calculating syndrome on the data of the coding block 1 which is inputted into the decoder, the syndrome calculation of the coding block 1 is completed at the time when the bit at the end of the coding block 1 is inputted. Next, error positions and error values will be calculated upon the basis of the calculated syndrome, however there is needed a specific calculation time (Tj) depending upon code format thereof, as well as a calculation circuit therefor. As a result of this, the time when starting the output of the corrected data of the coding block 1 is at a time point of counting up “time for 1 coding block+Tj” starting from a time point of inputting into the decoder, i.e., after the “Tm”. For the coding blocks flowing thereafter, there are also generated the delay “Tm”s in the same manner.
On the other hand, the delay will not occur under the condition of the decode OFF. This is because the data inputted into the decoder is outputted as it is, with the same phase thereof, and therefore it can be considered that a time relationship between the input data and the output data is kept to be equal to each other.
When the network management system or the operator exchanges or alters the decoding operation from OFF condition into ON condition, there occurs a jump in the phase corresponding to the decoding delay mentioned in the above. In more detail, said information is shifted backwards by the “Tm” on the time axis, therefore a portion of the signal is outputted in duplicate. For example, when exchange is made at a time point during the input of the coding block 2 (i.e., when outputting it at the same time), the data of the coding block 1 is outputted from on a_way thereof just after the coding block 2 is outputted until on the way thereof. Accordingly, the portion of data of the coding block 1, being outputted previously, and a portion of data of the coding block 2, being also outputted previously, (sum of both is equal to the Tm in time) are outputted, again (not shown in figure).
Also, on the contrary to this, when the decoding operation is exchanged from ON condition into OFF condition, there also occurs a jump in the phase, and said information is shifted forwards on the time axis, and therefore the signals are outputted with omission of a part thereof. For example, in FIG. 7, if it is exchanged at the time point when outputting the coding block 1 during the inputting of the coding block 2, a portion of data of the coding block 1 which has not yet been inputted, all data of the coding block 2, and a portion of data of the coding block 3 which was inputted previously (the sum of those three is equal to the Tm in time) will not be outputted. Accordingly, the coding block 3 is outputted from on a halfway thereof just after the data of the coding block 1 is outputted on the way thereof, thereby causing the loss or omission of the data.
In any event, the phase jump, for one (1) coding block at the lowest, can be observed at an upper client side (i.e., a side which receives the signal in down stream) through the exchange operation between ON and OFF in the decoding operation, and there would be caused a condition of instantaneous or abrupt interruption, such as a miss-synchronization of the frame, thereby bringing about large problems in reliability of the network.